Finished robot, on track.
I started with the same base as the Cocoabot http://letsmakerobots.com/node/36700 - I had not seen his, but great minds think alike. I built this as a demo bot for Roborave.org. I used aa Arduino pro328, with a proto shield jusrt for connections, and a protoboard on the top for my connections and two 2-motor H-bridge chips.
If you look at the track, you can see it is just black electrical tape. The original track - on the other side - has a tight turn the tracked base could not handle because it was too long.
The arm motor is to move an arm to dump ping-pong balls into a box. The front sensor determines the distance to the box. This is a QRD1114 IR LED and optotransistor.
The line followers are a Sharp GP2Y0D810Z0F. Thru a mixup, this sensor was meant to be the distance sensor. It is another IR LED/optotransitor, but with a filter that has a PWM-like output. I had to custom make a little PCB out of copper clad, JB weld the sensors onto them, then cut my own pads to solder the connections to. The "art" piece is my prototype connections to make sure I knew how to connect the thing.
Otherwise, construction was simple. I started with the electronics stack in the middle, but had to move it back to balance with the arm motor (a kit by the same folks as the base).The same battery pack powered everything.